Vehicles have been developed to perform engine stop at idle conditions when specific conditions are met and then to automatically restart the engine when restart conditions are met. Such idle-stop systems enable fuel savings, reduced exhaust emissions, reduced vehicle noise, and the like.
Engines may be stopped at a desired position (for example, when the piston of a particular cylinder is at a particular position) to improve the engine starting during a subsequent restart. In vehicles configured to perform idle-stop operations, wherein engine stops and restarts are repeated multiple times during a drive operation, stopping the engine at the desired position may provide for more repeatable starts.
One example approach for selecting an engine stop position during an idle-stop operation is shown by Takahashi et al. in US 2008/0092841. Herein, during the idle-stop operation, an engine control apparatus is configured to identify a cylinder that has stopped during an intake stroke of the cylinder and to further specify a desired piston stop position in the cylinder. During a subsequent restart operation, the control apparatus then specifies a fuel injection amount, based on the cylinder and piston stop position, to stabilize the torque generated at engine restart and reduce chances of misfire upon restart.
However, the inventors herein have recognized several potential issues with such a system. As one example, during the subsequent restart, it may be difficult to determine an accurate aircharge estimate. This may be largely due to uncertainties in determining the actual initial engine position. Additionally, uncertainties in the mass of air trapped in the selected cylinder, uncertainties in cylinder air temperature, uncertainties in the residual gas fraction in the cylinder, and/or uncertainties in the amount of charge that has leaked after intake valve closing may contribute to engine position errors and consequently inaccurate aircharge estimates. To compensate for the engine position and aircharge estimate errors, the engine controller may bias the fuel injection amount at restart towards fueling rich. Due to the frequency of restarts experienced in a vehicle operating with idle-stop systems, frequent engine restarts that are biased fuel-rich (due to position errors) may degrade engine fuel consumption. And since a large percentage of vehicle hydrocarbon emissions occur during engine restart (when the catalyst has not yet reached light-off temperature), such restart operations may also result in increased hydrocarbon emissions. As a specific example, even assuming a cylinder is positioned in an intake stroke, due to variable valve timing operation, the exhaust valve may be open, and or the intake valve may be closed.
Thus, in one example, some of the above issues may be addressed by a method of controlling a vehicle system including an engine that is selectively deactivated during engine idle-stop conditions and selectively reactivated during engine restart conditions, the method comprising, during an engine restart from an idle-stop, performing a first combustion event in a cylinder with a piston at an engine crankshaft position that is retarded from a crankshaft position at which said cylinder's exhaust valve closes and advanced from a crankshaft position at which the cylinder's intake valve closes, during a cycle of said cylinder. The method may further comprise adjusting a valve overlap period of the cylinder to a shorter crankshaft angle duration and retarding a timing for intake valve closing within combustion stability limits.
In one example, an engine controller may be configured to perform a first combustion event in a cylinder whose piston is in a position that conforms to predefined criteria (such as indicated above). The piston of the first firing cylinder may be selected at the specified engine crankshaft position during engine restart or before the engine is reactivated (for example, before completion of the previous idle-stop operation and/or before the first combustion event at restart). Alternatively, the piston may be positioned according to the selected criteria during the engine restart or before the engine is reactivated using a starter motor system. Once a cylinder has been selected where the piston is at an engine crankshaft position that is retarded from a crankshaft position at which the cylinder's exhaust valve closes, and near and advanced from a crankshaft position at which the cylinder's intake valve closes, and further upon adjusting a valve overlap period of the cylinder to a shorter crankshaft angle duration, while retarding the timing for intake valve closure, the first combustion event may be performed when the piston of the cylinder is near the top dead center (TDC) of a compression stroke of the engine. As such, the predefined criteria for positioning the piston of the cylinder may be selected such that a substantially accurate aircharge estimate may be made in the first firing cylinder. In this selected position, the amount of residuals in the cylinder may be reduced, and properties used to estimate and/or infer a cylinder piston position, such as the cylinder air temperature and pressure, may be better estimated. By reducing positioning errors, inaccuracies in cylinder aircharge estimation may be reduced, thereby also reducing air-fuel ratio errors. In doing so, the quality and repeatability of engine restarts may be significantly improved. Additionally, improvements in vehicle fuel efficiency and fuel emissions may also be achieved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.